The microelectronic revolution that began with the invention of transistors in the 1940s is now witnessing the incorporation of integrated circuits (ICs) into virtually every machine and appliance that people use daily, from cars and telephones to televisions and toasters. This large scale use of microprocessors in consumer electronics has been made possible in part by the development and manufacture of transfer molded plastic packages for ICs. Today, transfer molded plastic packages are used for more than ninety percent of all manufactured ICs and they continue to dominate the market for IC packages.
There are numerous advantages in using transfer molded plastic packages. They are small in size, easy to fabricate, can have large numbers of connecting leads, can be mass produced, and most importantly, can be made at low cost, typically only a few cents each in commercial quantities. This contrasts with costs of several dollars, typically, for higher performance packages, a cost which would prohibit the use of ICs in many consumer product applications.
A typical transfer molded plastic package consists of a lead frame, a semiconductor IC chip mounted on the lead frame, and an encapsulant enclosing the chip. The lead frame is the central supporting structure of the package. It normally consists of a baseplate for mounting the IC chip and many conductive strips (i.e. leads) extending outwardly from the baseplate. The lead frame is usually made by stamping or chemically milling a thin sheet of metal such as a copper alloy. Prior to encapsulation, the chip is electrically connected to the leads by connecting thin gold wires between the bonding pads of the chip and the leads. The chip, lead frame and wires are then encapsulated in a thermoplastic material, exposing only the outer ends of the leads for connection. Accordingly, the outer ends of the leads can be formed into various shapes for inserting into a printed circuit board or for surface mounting.
The above-described conventional transfer molded plastic packages are not suitable for high speed or high frequency operations such as those involving microwave frequencies. A microwave package requires low parasitic coupling between the input and output leads of the package, a low inductance ground path, a high degree of isolation of the chip, and the ability to dissipate heat generated by the chip. In conventional transfer molded plastic packages, the closely spaced leads generate coupling between the leads. Additionally, the encapsulant produces capacitive and resistive coupling between the input and output leads, and the capacitive coupling increases with increased operating frequency. Further, the encapsulant undesirably provides a parasitic capacitance between the leads and the chip, therefore reducing the electrical isolation of the chip, which is especially harmful for microwave circuit chips requiring high gain. Moreover, the encapsulant also loads the small spiral inductors normally used on a microwave circuit chip, thereby undesirably changing the properties of the microwave circuits. As a result, it precludes the use of such packages for complex multi-function microwave circuits.
Because conventional transfer molded plastic packages cannot be used for microwave circuits, other packages such as thin film or thick film ceramic packages are normally used. Compared with transfer molded plastic packages, ceramic packages are much more expensive to produce and are difficult to fabricate. Other alternative packages for microwave circuits such as commercially available TO-8 style packages are also much more expensive and generally cannot be surface mounted; they are therefore not suitable for applications that require surface mountable components.
Due to advances in microwave device and circuit technology, microwave circuits such as high performance GaAs Monolithic Microwave Integrated Circuits (MMICs) are being increasingly used in large numbers in applications such as cellular communications and Direct Broadcast Satellite (DBS) links. Some applications within these technologies have been foreclosed by lack of a commercially feasible low cost package for such circuits. Since the cost of GaAs microwave circuits has been drastically reduced in recent years due to the advancement of GaAs technology, the packaging cost of these circuits has increasingly become an important issue. Today, a packaged GaAs MMIC downconverter for satellite TV system costs only a few dollars or less; the cost of the package itself can significantly affect the cost of the packaged circuit and therefore the profitability and competitiveness of the product. By using lower cost packages, the cost of the packaged circuit can be greatly reduced. For example, by changing the package for a GaAs MMIC circuit from a ceramic leadless chip carrier package to a TO-5 package, the cost of the packaged circuit is reduced by 50 percent. However, TO-5 package is still much more expensive than the plastic packages. Accordingly, there has been a need, previously unmet, to provide a low cost, high performance, mass producible microwave package.